What do you mean? Number of tris and resolution?
Both as small as possible without destroying the look .
For medium important stuff I´d say around 500 tris and a resolution of 256*256. If you are talking about the scale, 1 unit = 1m does probably do a good job. Float should give enough precision than for small and for big objects, as long as you don´t have distances of lightyears or something .

If you´ve got any ideas for features, feel free to request them here: http://sourceforge.net/tracker/?group_id=318519&atid=1339401 (needs a sourceforge account, to add stuff)